As mobile devices such as mobile electronic devices and mobile communication devices have highly developed, non-aqueous electrolyte secondary batteries with higher energy density are recently needed to improve economic efficiency and reduce the size and weight of the devices. The capacity of the non-aqueous electrolyte secondary batteries of this type can be improved by known methods: use of a negative electrode material made of an oxide of B, Ti, V, Mn, Co, Fe, Ni, Cr, Nb, or Mo, or a composite oxide thereof (See patent document 1 and 2, for example); use of a negative electrode material made of M100−xSix (x≥50 at %, M=Ni, Fe, Co, Mn) subjected to melting and rapid cooling (patent document 3); use of a negative electrode material made of a silicon oxide (patent document 4); use of a negative electrode material made of Si2N2O, Ge2N2O, and Sn2N2O (patent document 5), and others.
Since silicon has a theoretical capacity of 3580 mAh/g far higher than a theoretical capacity of 372 mAh/g of a carbon material that is put in practical use at the present time, it is a material most expected in miniaturization and higher capacity of a battery. Silicon is known in various forms different in a crystal structure according to a manufacturing method thereof. For example, patent document 6 discloses a lithium ion secondary battery that uses single crystal silicon as a support of a negative electrode active material. Patent document 7 discloses a lithium ion secondary battery that uses a lithium alloy LixSi (x: 0 to 5) using single crystal silicon, polycrystalline silicon, and amorphous silicon. According to the document, LixSi that uses amorphous silicon is particularly preferred, and pulverized crystalline silicon covered with amorphous silicon obtained by plasma decomposition of silane gas is illustrated.
The negative electrode material can be endowed with conductivity by known methods: mechanical alloying metal oxide such as silicon oxide with graphite, followed by carbonization (patent document 8); coating the surface of a Si particle with a carbon layer by chemical deposition method (patent document 9); coating the surface of a silicon oxide particle with a carbon layer by chemical deposition method (patent document 10). These methods can improve the conductivity by the carbon layer provided onto the particle surface, but cannot relax the large volume change due to charge/discharge, and cannot prevent degradation of ability to collect current and cycle performance reduction due to the large volume change, which are problems of silicon negative electrodes.
Accordingly, in recent years, the following methods are disclosed: a method where a usage rate of silicon in battery capacity is restricted to suppress volume expansion (patent documents 9, 11 to 13); and as a method where a grain boundary of polycrystalline particles is used as a buffering region of the volume change, a method where a silicon melt in which alumina is added is quenched (patent document 14), a method using polycrystalline particles composed of mixed phase polycrystals of α, β-FeSi2 (patent document 15), and a high-temperature plastic forming process of a single crystal silicon ingot (patent document 16).
Methods to design a laminating structure of the silicon active material to relax volume expansion are also disclosed, for example, a method to arrange silicon negative electrodes into two layers (patent document 17), a method to suppress the collapse of particles by coating or encapsulating with carbon, other metal or oxide (patent documents 18 to 24). In direct vapor growth of silicon onto a current collector, a method controlling the growth direction to suppress the lowering of cycle performance due to volume expansion is also disclosed (patent document 25).
As described above, silicon, silicon alloy, and silicon oxide have been investigated as a negative electrode active material. However, it has not been proposed a practical negative electrode active material which fulfills following features: volume change due to occluding and emitting Li is sufficiently suppressed, lowering of the conductivity due to separating from a current collector or fine pulverization caused by breakage of particles can be relaxed, mass production is possible, and is cost-favorable.
On the other hand, it is disclosed that composite oxides composed of elements of SiOC, being formed by baking methylsiloxanes such as silane, silicone oil, and silicone resin at a high temperature, has a charging or discharging capacity (patent documents 26 to 29). Although the discharging capacity is 500 mAh/g or so, the cycle performance is excellent. However, there has been a disadvantage that the energy density is not increased since the irreversible capacity is extremely high and the discharge curve changes linearly. Accordingly, some methods has been proposed: adding metal Li to the composite oxide and reacting chemically to dissolve the irreversible capacity; or adding metal silicon, which has low irreversible capacity, to increase the battery capacity, thereby increasing the capacity efficiency by weight (patent documents 30, 31). By these methods, however, sufficient property for practical use cannot be exhibited.